Papermaking is a sophisticated operation involving massive and very expensive machines. These machines are increasingly running at higher speeds, meaning that their overall efficiency must be very high, and in particular, the efficiency of their sub-components must also be very high. The papermaking process requires that water be removed from the initial pulp fiber solution as the paper is formed. The pulp fiber solution, once in the drying section of a papermaking machine, is referred to as the paper web. The paper web is supported as it travels across the machine following a path during which moisture is progressively removed therefrom. The support is provided by endless sheets of porous fabric, felts, wires or other water and gas permeable support means, all of which are generically referred to as the  less than  less than felt or felts greater than  greater than  in the description and appended claims.
The paper web travels from what is referred to as the wet end of the machine to the dry end thereof. In its path, the paper web runs over numerous heated drying cylinders where moisture is evacuated therefrom either by direct evaporation or transfer of moisture to the felts or to the surface of the drying cylinders. A network of ventilator devices is used throughout the drying section in order to inject heated dry air at numerous locations and promote the removal of moisture from the papermaking machine. At the dry end, the machine outputs the resulting paper, which is then generally reeled to be shipped elsewhere.
Papermaking machines can be built according to numerous possible configurations. One configuration in particular is the twin-wire draw, where the papermaking machine comprises two superposed rows of axially-parallel and horizontally-disposed heated drying cylinders. The paper web runs in a serpentine or zigzag path where it defines loops by alternating between the two rows of drying cylinders as it advances along the drying section. The paper web is being supported in most of its path with the assistance of the felts. There is generally one felt for each row of drying cylinders. Each felt presses the paper web on a portion of the surface of the drying cylinders of the corresponding row. Each felt also runs over a felt roll between each pair of adjacent drying cylinders of a same corresponding row. The felt rolls are located deep in the space between the two adjacent drying cylinders. This configuration allows to maintain the paper web in supporting contact with the felt as long as possible. The felt rolls essentially redirect a felt to the next drying cylinder of the same row.
It should be noted at this point that the terms  less than  less than roll or rolls greater than  greater than  and  less than  less than cylinder or cylinders greater than  greater than  are synonyms since both are elongated members with a circular cross-section, the only distinctions in the present context being that the drying cylinders are generally much larger in diameter than the felt rolls and are heated by appropriate means that are well known in the art. The segregated use of the terms in the text is only for the purpose of clarity. The  less than  less than cylinder or cylinders greater than  greater than  are sometimes referred to as  less than  less than drum or drums greater than  greater than  in other documents.
Traditional drying cylinders and felt roll arrangements feature the felt roll being positioned intermediate to the axis of rotation of adjacent drying cylinders, thus halfway between two adjacent drying cylinders. Inherent in these arrangements is the fact that the paper web is repetitively unsupported wherever the felt separates from the paper web to pass over a felt roll.
In order to increase the speed of papermaking machines, it was desirable to reduce the length of unsupported paper web because of the risks of rupture thereof, particularly near the wet end where the, paper web is weaker. One solution to this problem was to move the axis of rotation of the felt rolls backwards, more particularly towards the wet end of the machine, as illustrated in FIG. 1. This offset configuration has resulted in the felts and the paper web being in contact longer, thus minimizing the length of unsupported paper web as it travels from one drying cylinder to another.
The offset configuration of the felt rolls has also resulted in creating what is known as offset pocket spaces. In FIG. 1, the offset pocket spaces (12) are identified as hatched areas. Each offset pocket space (12) is situated between three successive drying cylinders (20) over which consecutively runs a paper web (14). There are thus many offset pockets (12) since there are many groups of three successive drying cylinders (20). A pocket space (12) may be roughly defined as the space limited by a felt (16) between two successive drying cylinders (20) of a same row, a first draw of paper web (14) from a first drying cylinder (20) to the next drying cylinder (20) of the other row, a second draw of paper web (14) between that second drying cylinder (20) and a third successive drying cylinder (20) on the same row as the first one, and the free surface of the second drying cylinder (20). The pocket spaces (12) are only open at each side of the machine. It should be noted that the pocket ventilators have been omitted from FIG. 1 to simplify the drawing.
Unfortunately, ventilating offset pocket spaces is more difficult than in symmetrical pockets spaces found in traditional configurations. This results from the reduction of the length of felt which does not support the paper web on the side upstream of the felt roll. Since the felt is permeable to air and the paper web is not, the conventional dry heated air ventilators which were hitherto provided cannot be used the same way since air cannot be blown through the felt when it is supporting the paper web. Moreover, an offset pocket space is often too small for receiving a ventilating device therein. All of this has resulted in decreased air flow into offset pocket spaces, thus a decrease in the efficiency of the ventilation.
An example of a pocket ventilator previously known in the art is disclosed in U.S. Pat. No. 5,074,278 to Turcotte and issued Dec. 24, 1991. It illustrates a traditional symmetrical arrangement of pocket spaces. This patent is hereby incorporated by reference.
The geometry of a pocket and the natural air currents generated in the pocket space by the moving paper web and felt, as well as the high rotation speeds of the drying cylinders and the felt rolls, are key factors which increase the difficulty in ventilating a pocket space. Air introduced in a pocket space has a natural tendency to follow the movement of the felt and also tends to be trapped in a cleft defined when the felt rejoins a drying cylinder. Such cleft, known as a closing nip, is a zone of positive air pressure where air tends to flow through the felt to evacuate the pocket space. Furthermore, a zone of negative air pressure is created in a cleft defined where the paper web leaves the felt roll on its way to the next drying cylinder. Some air evacuates the pocket space to satisfy the negative pressure created therein at the opening nip. However, these natural air currents do not provide an adequate ventilation of the pocket space as they do not effectively sweep of the pocket space. As a result, air becomes entrapped therein, thereby increasing the humidity level within the pocket space and decreasing the overall drying capacity of the papermaking machine.
The main object of the present invention is thus to provide a solution to the above-discussed ventilation problems so that offset pocket spaces of the drying section of a papermaking machine could be adequately ventilated.